Drugs of abuse, such as heroin and morphine, have exacted an incalculable toll on society. Our understanding of the effects of opiates is still limited, in part due to an incomplete knowledge of the neural circuitries affected by morphine. Several lines of evidence implicate the medial thalamus, especially the midline-intralaminar nuclei, in the subjective effects of opiates sought by drug abusers. Rich in mu opiate receptors, the midline-intralaminar thalamic nuclei regulate affective responses to visceral stimuli, relaying this information to specific regions of the striatum, a brain region well known to play an important role in drug abuse. Thus, thalamo-striatal neurons are a key to understanding effects of opiates on the brain. The long-term goal of this application is to test the hypothesis that morphine binds to mu opiate receptors found on GABAergic terminals of the medial thalamus, thus disinhibiting glutamatergic thalamo-striatal neurons. The resulting activation of striatal glutamate receptors stimulates expression of immediate-early genes, which then participate in the synaptic remodeling and neural plasticity of drug abuse. This hypothesis predicts that microinfusion of a mu receptor antagonist into the medial thalamus will block the striatal c-Fos expression to systemic morphine (Specific Aim number 1, part A); medial thalamic microinfusion of morphine will induce c-Fos expression in the striatum (Specific Aim number 1, part B); augmentation of thalamic GABA activity will block the striatal c- Fos response to systemic morphine (Specific Aim number 1, part C), and intrathalamic morphine will induce c-Fos in the same set of striatal neurons activated by systemic morphine (Specific Aim number 1, part D). If morphine activates thalamo-striatal neurons, then c-Fos should be expressed by thalamic neurons retrogradely labeled from the striatum (Specific Aim number 2). Mu opiate receptors should be co-localized with GABA in synapses of the medial thalamus (Specific Aim number 3). Following systemic morphine treatment, Fos-positive neurons should be post- synaptic to input from the thalamus, determined by combining anterograde tracing with localization of morphine-induced c-Fos in individually-injected neurons of the striatum (Specific Aim number 4). Since thalamo-striatal neurons are glutamatergic, morphine should induce c-Fos expression in striatal neurons that contain specific glutamate receptor subunits (Specific Aim number 5). This will be approached by double immunocytochemistry for c- Fos and glutamate receptor subunits, and by RNA profiling from individual striatal neurons categorized by morphine-induced c- Fos. These studies should greatly extend our understanding of the role of thalamo-striatal circuits in the actions of morphine on the brain.